In the internal combustion engine, it is necessary to maintain the air-fuel ratio of the air fuel mixture at a predetermined value of a theoretical air-fuel ratio in order to reduce exhaust emission pollutants from the engine and to improve fuel consumption. It is, however, difficult to constantly maintain the air-fuel ratio at a predetermined value by means of only the operation of the carburetor. It has, therefore, been proposed that a device for correcting the air-fuel ratio be added to the carburetor so as to control the air-fuel ratio to a predetermined value. For example, the apparatus disclosed in Japanese patent Laid-Open publication No. 79830 of 1976 includes an air induction bypass passage for bypassing a throttle valve of the carburetor, an electromagnetic valve provided in the air induction bypass passage, an oxygen concentration cell type sensor (a so-called O.sub.2 sensor) provided in an exhaust pipe, and an electronic control unit receiving electrical signals from the O.sub.2 sensor and actuating the electromagnetic valve, whereby the air-fuel ratio predetermined to be lean can approach the theoretical air-fuel ratio by means of controlling the degree of opening of the electromagnetic valve in response to the electrical signals from the O.sub.2 sensor and by means of controlling the rate of air flowing through the air induction bypass passage.
In the above-mentioned prior art, however, the air-fuel ratio can be controlled by the O.sub.2 sensor provided in the exhaust pipe. That is, the rate of air flowing through the air induction bypass passage is controlled in response to the electrical signals from the O.sub.2 sensor provided in the exhaust pipe. In controlling the air-fuel ratio, therefore, a response time lag cannot be avoided. In order to correct the response time lag, it will be necessary to allow for a response lag in time for controlling the air-fuel ratio. It is, accordingly, difficult to optimally control the air-fuel ratio in response to engine driving conditions changing momentarily since the control accuracy will be reduced.